Intact motor nerve terminals sprout in response to paralysis or partial denervation and re-establish functional synaptic connections with inactive muscle fibers, thus insuring the maintenance of muscle innervation and strength. This remarkable feature of neuromuscular plasticity has been studied intensively, both because it is essential to our understanding of synapse formation and maintenance and because it provides a potential avenue for promoting repair of nerve injuries. However, the mechanisms underlying this type of neural plasticity are poorly understood. We and others have recently shown that the terminal Schwann cells (tSCs) that cap the nerve terminal play a central role in this process, by extending processes that trigger terminals to sprout and that guide these sprouts to inactive muscle fibers. We now propose to identify the molecules that activate quiescent tSCs to extend processes and the molecules that activated tSCs use to elicit sprouting. Aim 1 will test the hypothesis that reduced neurotransmitter release, and subsequent inactivation of muscarinic signaling, activates tSCs. Pharmacological inhibition of this muscarinic signaling in vivo and analyses of knockout mouse lines that are defective in muscarinic signaling will be used. Aim 2 will assess the role of tSCs in mediating growth factor induced sprouting. We will test this by exogenous application of IGFs and CNTF, two growth factors known to elicit sprouting, to determine whether these agents activate tSCs to extend processes. We will then examine CNTF -/- mice, whose nerve terminals apparently do not sprout in response to partial denervation or botulinum toxin, to determine whether their tSCs are defective in their response to normal sprouting stimuli. In Aim 3, we will test the hypothesis that NCAM and N-cadherin, the two cell adhesion molecules (CAMs) upregulated in activated tSCs, elicit sprouting. We will exploit the ballistic transfection technique to overexpress these CAMs selectively in quiescent tSCs in vivo, and examine the incidence of terminal sprouting. Secondly, we will transplant fibroblasts genetically modified to express the CAMs to intact endplates, to determine whether these fibroblasts acquire the ability to induce terminal sprouting. Through this work, we attempt to further define the role of Schwann cells in the reactive sprouting and to gain new insights into the inter- and intracellular signaling associated with terminal sprouting.